1. Field of the Invention
The present invention relates to the field of backlight driving of liquid crystal displays, and in particular to a backlight drive circuit with dual boost circuits.
2. The Related Arts
Nowadays, science and technology are in rapid progress and various innovated information devices are available in the market to suit the needs of general people. In early days, displaying devices are mostly CRT (Cathode Ray Tube) displays, which have a bulky size and consumes a large amount of electrical power and generate radiation so that long term use of the displays is hazardous to body health for users. Thus, the displays that are currently available in the market are gradually switched from liquid crystal displays (LCDs) that substitute the CRT displays used in early days.
LCDs have a variety of advantages, such as thin device body, low power consumption, and being free of radiation, and are thus widely used. Most of the LCDs that are currently available in the market are backlighting LCDs, which comprise a liquid crystal panel and a backlight module. The operation principle of the liquid crystal panel is that liquid crystal molecules are interposed between two parallel glass substrates and a drive voltage is applied to the two glass substrates to control the liquid crystal molecules to change direction in order to refract out light emitting from the backlight module for generating images. Since the liquid crystal panel itself does not emit light, light must be provided by the backlight module in order to normally display images. Thus, the backlight module is one of the key components of an LCD. The backlight module can be classified in two types, namely side-edge backlight module and direct backlight module, according to the location where light gets incident. The direct backlight module comprises a light source, such as a cold cathode fluorescent lamp (CCFL) or a light-emitting diode (LED), which is arranged at the back side of the liquid crystal panel to form a planar light source that directly provides lighting to the liquid crystal panel. The side-edge backlight module comprises a backlighting source of an LED light bar arranged at an edge of a backplane that is located rearward of one side of the liquid crystal panel. The LED light bar emits light that enters a light guide plate (LGP) through a light incident face of the light guide plate and is projected out through a light emergence face of the light guide plate, after being reflected and diffused, to thereby pass through an optic film assembly to form a planar light source for the liquid crystal display panel.
With the progress of science and technology and the increasing demand for material of the modern people, the LCDs are now made with increasingly expanded sizes and the size of the liquid crystal display panels is also getting larger and larger, whereby the needed LED backlighting brightness is also getting greater and greater. Referring to FIG. 1, which is a circuit diagram of a conventional LED backlight drive circuit with a single boost circuit, which comprises an electrical inductor L10, a MOS transistor Q10, a rectifier diode D10, and an output electrolyte capacitor C10 to form a single boost circuit that provides a higher level of voltage to an LED light string 100, wherein the magnitude of the output voltage is related to the duty ratio (D) of a gate drive signal of the MOS transistor Q10. The output voltage Vo=Vin(1−D) and the greater the duty ratio D is, the greater the output voltage Vo will be. However, increasing the duty ratio D will extend the conduction time of the MOS transistor 10 in a cycle, whereby the electrical current in the electrical inductor L10 linearly increases so that the current quickly raises. This current also flows through the MOS transistor Q10. The power consumed in the electrical inductor L10 and the MOS transistor Q10 is increased and temperature rises. When the electrical current through the electrical inductor L10 exceed the maximum level of current that the electrical inductor L10 and the MOS transistor Q10 can take, the electrical inductor L10 or the MOS transistor Q10 will get burnt down. Due to the limitation imposed on the specifications of the components of the circuit, a single boost circuit can provide a power that is limited (around 70 W) and does not suit the needs for development of liquid crystal displays.